[clang] [clang][RISCV] Update vcpop.v C interface to follow the nameing convention (PR #94318)

[Pengcheng Wang via cfe-commits]

wangpc-pp wrote:

This seems to be a breaking change, do we need to create issue in https://github.com/riscv-non-isa/rvv-intrinsic-doc?

https://github.com/llvm/llvm-project/pull/94318